1. Field of the Invention
The present invention relates to a flexible printed circuit board (FPCB), and more particularly, to an FPCB which prevents its conductive layer from being short-circuited when it is subject to repetitive bending.
2. Description of the Related Art
An FPCB is a circuit board which can be easily bent and is typically formed by laminating a circuit pattern to a base film of a suitable resin known in the art. Such an FPCB is widely used to connect various parts of electronic equipment.
FIG. 1 shows an apparatus disclosed in Korean Patent Publication No. 1999-31984. In FIG. 1, a liquid crystal display (LCD) device includes an upper substrate 22 and a lower substrate 21 connected to the upper substrate 22, and an FPCB 30 is connected to the LCD device. A thin-film transistor and a first electrode driven by the thin-film transistor are formed in an upper portion of the lower substrate 21, and a second electrode is formed in a lower portion of the upper substrate 22.
A liquid crystal material is disposed within the LCD device between the first and second electrodes. In operation, the LCD device is typically subject to a potential difference applied between the first and second electrodes, which cause a disturbance in the orientation of the constituent liquid crystal material. Accordingly, a particular image or light pattern may be appeared in response to the spatial arrangement of the liquid crystal material. A terminal of the thin-film transistor is typically connected to a driving circuit by the FPCB 30. In addition, a driving integrated circuit (IC) 23 related to the driving of the first and second electrodes may be installed on the lower substrate 21.
FIG. 2 is an enlarged view of the portion II shown in FIG. 1 illustrating a cross-section of the FPCB 30 in detail. The FPCB 30 includes an end portion 30a, a bendable portion 30b extending from the end portion 30a, and a circuit portion 30c extending from the bendable portion 30b. A conductive layer 32 is formed on an inner surface 31″ of a base film 31, covering substantially the entire surface thereof. Another conductive layer 33 is formed on an outer surface 31′ of the base film 31, substantially covering the circuit portion 30c. The conductive layer 33 is electrically connected to the other conductive layer 32 through a via-hole or connection hole 34.
A plating layer 36 formed of nickel-gold or tin is disposed on the first conductive layer 32 and covers the end portion 30a and the bendable portion 30b in order to prevent oxidization of the first conductive layer 32, thereby improving electric contact. In order to further prevent the first and second conductive layers 32 and 33 from being cracked, oxidized, scratched, or short-circuited with an electric device, cover layers 35b and 35a are formed on the first and second conductive layers 32 and 33, respectively, and covers basically the circuit portion 30c where the plating layer 36 is not formed. The first and second conductive layers 32 and 33 are formed of a metal such as copper and comprise a predetermined circuit for driving the electronic device. Typically, the first and second conductive layers 32 and 33 are connected to each other through a single via-hole.
The first conductive layer 32 is required to be electrically connected to a terminal 24 of an electronic device (e.g., the LCD device) as shown in FIG. 2. Usually, since a gap between conducting lines in the first conductive layer 32 is very narrow, the first conductive layer 32 is electrically connected to the terminal 24 by an anisotropic conductive film (ACF) 25.
FPCB may be subject to repetitive bending, for example, in order to connect the FPCB 30 having the above-described structure to the terminal 24 of the LCD device during a manufacturing process of the device, the bendable portion 30b of the FPCB 30 needs to be repeatedly folded and unfolded. This generates a repetitive compressive and tensile stress onto the conductive layer 32, and as a result, a crack C is often developed particularly at a portion A of the first conductive layer 32 where the cover layer 35b ends. The crack C becomes larger as such a folding and unfolding are repeated and eventually short-circuits the first conductive layer 32. Consequently, a fundamental purpose of the FPCB 30 for providing connection between two parts (e.g., the terminal 24 of the LCD device and a terminal of a driving circuit of the LCD device) cannot be accomplished.
To overcome this problem, the cover layer 35b may be further extended to cover substantially the entire portion of the first conductive layer 32 and the plating layer 36 of the bending portion 30b. However, this arrangement hinders the subsequent bonding of the ACF 25 with the plating layer 36.